Hydrogen absorption pressure-composition (P-C) isotherms of C15 Laves phase Mm(x)Tb(1-x)CO(2) (x = 0, 0.05, 0.1, 0.15, 0.2) alloys have been investigated by pressure reduction method using Sieverts' apparatus, in the ranges 30 <= T degrees C <= 200 and 0.0 1 <= P (bar) <= 20, respectively. The P-C isotherms show the presence of a single plateau region in the temperature and pressure ranges studied. The dependences of the plateau pressure at any given temperature and the hydrogen storage capacity at any particular pressure and temperature on the Mischmetal (Mm) content have been discussed. The dependence of the thermodynamics of dissolved hydrogen in these alloy hydrides on the hydrogen concentration has been studied. The absorption of hydrogen is accompanied by a maximum of 16-18% unit cell volume expansion in the host material without any structural changes as evidenced by the powder X-ray diffraction studies of alloy hydrides. Instead, hydrogen induced amorphization (HIA) at higher hydrogen concentration has been observed. The dependence of HIA on Mm content has been discussed. In addition, the kinetics of hydrogen absorption have been studied in the temperature range 50 <= T degrees C <= 200 and activation energy for the alpha + beta and beta phase and the diffusion coefficient in the beta phase have been calculated from the hydrogen absorption kinetics and the results have been discussed. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.